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Show 1888.] ANATOMY OF THE MESOSUCHIA. 427 rank as a rib; and (2) of a much smaller component which, descending from the neural arch upon the upper surface of the sacral rib, thins out on this, and ends at a short distance from the base of this. The long sacral ribs have a strong downward slant. Their distal end is dilated, thus increasing the extent of the iliac articulation. In a cross section through the middle of a sacral rib, the vertical exceeds the horizontal diameter. The rib of the first sacral vertebra is attached by a large base to the lateral surface of the centrum close to its anterior end, but it does not contribute any portion to the anterior, terminal, articular surface of the centrum. The neurapophysis encroaches slightly on this surface. The anterior terminal surface of the first and the posterior terminal surface of the second sacral vertebra are distinctly concave. Their dimensions exceed those of the applied surfaces of the vertebrae, which are nearly plane, with a slight central depression. The rib of the second sacral vertebra is attached to the side of the centrum nearly equidistantly between the two ends, and no part of the costal suture approaches either terminal surface of the centrum. The spinous processes are tall, their antero-posterior extent is less than in the thoracic vertebrae. Caudal Vertebra; (Plate XVIII. fig. 5).-Their centrum is laterally compressed. The lower border of the posterior terminal surface is truncated by a double chevron facet. Their transverse process, present in the front part of the tail, ossifies independently of the centrum with which it is suturally connected, and thus is morphologically a rib. Pectoral Girdle. The series of remains bearing the Cat. No. 30 comprises both coracoids and the right humerus. Both scapulae are preserved in series No. 31. Unfortunately no series contains both the coracoid and the scapula. Scapula (fig. 1, p. 428).-Of the right only the ventral half is preserved. The left comprises the whole bone, but both its ends are defective. Together they give the form of the entire bone. This is broader, shorter, stouter, and flatter than the scapula in extant Crocodiles. The shaft is short and contracted. Above this narrow part the antero-posterior dimension rapidly augments, chiefly by the backward inclination of the posterior border. The outline of the dorsal extremity is an arc of a large circle, The ventral end is deeply indented by a notch which separates off a stout posterior part (g c) from a thin flattened process (ps), which in the articulated skeleton appears to have been directed downwards and forwards. This latter appears to correspond to the process termed acromial on the anterior border of the scapula in some Anomodonts, and it suggests the presence of a precoracoid element in these Mesosuchia, of which the Eusuchia do not retain any trace. The stout process (g c) is subdivided into :-(g) a posterior subcircular, smooth, hollowed portion, obviously the scapular component of the glenoid fossa ; and |
